Throughout the immigration process, you’re going to be handling a lot of documents.
It can be hard to know when you need to fill out the form—the G-325a is typically sent with other forms. In the marriage and fiancé visa context, both the U.S. citizen and immigrant fiancé need to fill out the forms.
If you want to bring your alien fiancé into the United States, you will need to file a petition with the USCIS. The petition is called the I-129F Petition for Alien Fiancé and can be found on USCIS.gov.